The Friction Stir Welding (FSW) process is a solid-state based joining process, which makes it possible to weld a wide variety of materials (Aluminum, Copper, Stainless Steels, etc.) to themselves and to weld various combinations (e.g. aluminum alloys 6xxx/5xxx, 2xxx/7xxx, etc.) to each other. The process is based on “plunging” a rotating FSW tool into the joining area. The rotating friction stir welding tool heats the workpiece(s) by friction, so the material becomes plasticized and flows around the axis of the tool due to shear caused by the rotating tool.
FIG. 1 shows a prior art friction stir welding tool 10. The tool 10 includes a pin 12 which, preferably, is threaded. The shank 18 is for gripping in a chuck or collet of a friction stir welding machine, so that tool 10 can be rotated. While tool 10 is rotating, pin 12 is pressed against the workpiece(s) which is/are to be welded. Friction between the workpiece(s) and pin 12 causes the material of the workpiece(s) to become heated to its softening temperature and thus plasticized.
Pressure between pin 12 and the plasticized workpiece(s) causes pin 12 to be plunged into the workpiece(s). Friction between the pin 12 and the workpiece(s) causes plasticized workpiece material to flow around the axis of pin 12 and thus welding occurs without melting. The shoulder 14, having workpiece engaging surface 16, is for preventing material from flowing upwardly, out of the joint being formed. The tool is rotated in the direction such that the threads 13 on pin 12 push plasticized material downwardly into the joint. Optionally, pin 12 further has flats 15 which reduce the torque required to rotate friction stir welding tool 10.
Preferably, workpiece engaging surface 16 of shoulder 14 has a spiral thread 17 which tends to move plasticized material inwardly, toward the proximal end of pin 12, when tool 10 is rotated in the direction for which the threads 13 on pin 12 push plasticized material downwardly into the joint.
Since the workpiece material(s) being friction stir welded is/are heated to such a high temperature by friction with the tool that the workpiece material(s) becomes softened or “plasticized”, while the tool, to remain dimensionally stable, must not become plasticized, it is necessary for the tool to have a higher operating temperature range than the workpiece(s). The tool may be comprised of a very hard metal having good high temperature properties, or it may be comprised of a cement or a ceramic material. However, such materials are generally subject to failure by fracture.
Tool failure thus puts limitations on the materials which can be welded by friction stir welding, and also puts limits on the speed with which a friction stir welding tool can be caused to move along a joint being welded. It is desirable to increase friction stir welding speeds both from the standpoint of production, and also because faster welding results in a narrower and stronger heat affected zone in the weldment. There is thus a need for improved friction stir welding tools which are more fracture resistant than present day friction stir welding tools.